In a cellular communication system, each of the subscriber units (typically mobile stations) communicates with typically a fixed base station. Communication from the subscriber unit to the base station is known as uplink and communication from the base station to the subscriber unit is known as downlink. The total coverage area of the system is divided into a number of separate cells, each predominantly covered by a single base station. The cells are typically geographically distinct with an overlapping coverage area with neighbouring cells. FIG. 1 illustrates a cellular communication system 100. In the system, a base station 101 communicates with a number of subscriber units 103 over radio channels 105. In the cellular system, the base station 101 covers users within a certain geographical area 107, whereas other base stations 113, 115 cover other geographical areas 109, 111. Some overlap areas 117 can be covered by more than one cell.
As a subscriber unit moves from the coverage area of one cell to the coverage area of another cell, the communication link will change from being between the subscriber unit and the base station of the first cell, to being between the subscriber unit and the base station of the second cell. This is known as a handover. Specifically, some cells may lie completely within the coverage of other larger cells.
All base stations are interconnected by a network. This network comprises communication lines, switches, interfaces to other communication networks and various controllers required for operating the network. The base stations themselves can also be considered part of the network. A call from a subscriber unit is routed through the network to the destination specific for this call. If the call is between two subscriber units of the same communication system the call will be routed through the network to the base station of the cell in which the other subscriber unit currently is. A connection is thus established between the two serving cells through the network. Alternatively, if the call is between a subscriber unit and a telephone connected to the Public Switched Telephone Network (PSTN) the call is routed from the serving base station to the interface between the cellular mobile communication system and the PSTN. It is then routed from the interface to the telephone by the PSTN.
Known cellular communication systems such as GSM uses discontinuous transmission whereby the transmissions from and to a base station is reduced when there is no voice activity on the link, for example during pauses in natural speech or when the other party is speaking. This significantly reduces the total power transmitted and thus reduces battery power drain and the interference caused to other subscriber units.
Conference calls are widely used in conventional landline telephony and are also becoming an increasingly popular way of conducting meetings using mobile telephony.
In a conference call, parties to the call may be calling from many different environments, some of which may have a high back ground noise level (a particular case is a mobile call from a moving car).
Discontinuous transmission (known as DTX for GSM) is in GSM implemented by first detecting that speech is not being transmitted then transmitting a silence descriptor frame at intervals of about 9 speech frames. Silence descriptor frames are widely used in cellular communication systems employing discontinuous transmission and contain information related to the background noise in the absence of voice activity. This enables the voice decoder to generate background noise corresponding to the background noise of the originating subscriber unit. This is very useful in two way communication, as the other party by hearing the background noise of the remote subscriber unit is aware that the link is still established and that the call has not been dropped. However, although DTX reduces the power transmitted, it also performs the function of simulating a high level of background noise at the far end of the link. This is disadvantageous in a conference call as the total background noise from all participants may significantly reduce the perceived quality. Thus, within a conference call DTX does not help the perceived quality of the multi party call.
Conference bridges typically operate a function to mute to some extent calling parties that are not active on the basis of sound volume. Typically, such a conference bridge is operated as part of the PSTN and as the sound volume of the background noise (especially from a mobile station) may be quite significant, the background noise may not be muted and the active speech signal generated may be disturbed by the background noise from the non-active participants.
Hence, there is a need for an improved method and system for performing conference calls in a communication system employing discontinuous transmission.